Are you a Coke or Pepsi drinker? Do you pull into McDonald's golden arches or prefer to "have it your way" at Burger King? When it comes to toothpaste, which flavor gets you brushing, Colgate or Crest? If you think it's just your taste buds that guide these preferences, you may be surprised by what neuroscientists are discovering when they peer inside the brain as it makes everyday choices like these.

Don't worry--no one's scanning your head as you stand in front of the beverage aisle or sit in line at the drive-through. Instead, brain scientists are asking volunteers to ponder purchasing choices while lying inside high-tech brain scanners. The resulting real-time images indicate where and how the brain analyzes options, weighs risks and rewards, factors in experiences and emotions and ultimately sets a preference. "We can use brain imaging to gain insight into the mechanisms behind people's decisions in a way that is often difficult to get at simply by asking a person or watching their behavior," says Dr. Gregory Berns, a psychiatrist at Emory University.

To scientists, it's all part of the larger question of how the human brain makes decisions. But the answers may be invaluable to Big Business, which plowed an estimated $8 billion in 2006 into market research in an effort to predict--and sway--how we would spend our money. In the past, marketers relied on relatively crude measures of what got us buying: focus-group questionnaires and measurements of eye movements and perspiration patterns (the more excited you get about something, the more you tend to sweat). Now researchers can go straight to the decider in chief--the brain itself, opening the door to a controversial new field dubbed neuromarketing.

For now, most of the research is purely academic, although even brain experts anticipate that it's just a matter of time before their findings become a routine part of any smart corporation's marketing plans. Some lessons, particularly about how the brain interprets brand names, are already enticing advertisers. Take, for example, the classic taste test. P. Read Montague of Baylor College of Medicine performed his version of the Pepsi Challenge inside a functional magnetic resonance imaging (fMRI) machine in 2004. Montague gave 67 people a blind taste test of both Coke and Pepsi, then placed his subjects in the scanner, whose magnetic field measures how active cells are by recording how much oxygen they consume for energy. After tasting each drink, all the volunteers showed strong activation of the reward areas of the brain--which are associated with pleasure and satisfaction--and they were almost evenly split in their preferences for the two brands. But when Montague repeated the test and told them what they were drinking, three out of four people said they preferred Coke, and their brains showed why: not only were the reward systems active, but memory regions in the medial prefrontal cortex and hippocampus also lit up. "This showed that the brand alone has value in the brain system above and beyond the desire for the content of the can," says Montague. In other words, all those happy, energetic and glamorous people drinking Coke in commercials did exactly what they were supposed to do: seeped into the brain and left associations so powerful they could even override a preference for the taste of Pepsi.